1. Field of the Invention
The present invention relates to an image processing apparatus, having a single-color image reproduction method and a multi-color image reproduction method, that outputs gradational images by digital image processing.
2. Description of the Related Art
In the prior art, digital electrophotographic apparatus processes single color gradations of an input image signal.
Referring to FIG. 1 showing this apparatus, processes of reproducing a single color image will be explained.
In FIG. 1, brightness signals of an image are obtained by a CCD 21, and are provided to a shading circuit 22 which corrects for variations in the sensitivity of each CCD element. To convert corrected brightness signals to density signals, the signals are processed by a LOG converter 23. The density signals are provided to a look-up table (LUT) 24 which compensates for gradational conversion due to influences of electrophotographic development characteristics. A pulse width modulator 25 converts the density signals into signals corresponding to dot widths, which are sent to a laser driver 26 for exposing a print medium (not shown).
According to the above-described digital signal processing, a latent image having gradational characteristics caused by changes in the dot area is formed on a photosensitive body (not shown) by laser scanning. A gradational image is obtained, following development, transferring and fixing.
When a full-color gradational image is reproduced, the additional processes of color separation, removal of a base color, masking, etc. are performed. Although a plurality of color signals undergo parallel processing, the method of reproducing the gradation of each color is the same as for a single color as explained above.
In the above-described prior art, however, brightness signals are usually input in units of eight bits, i.e., 256 different gradations, because of the limited brightness separation levels of the CCD 21.
FIG. 2 is a diagram in which brightness signals undergo a density LOG conversion. As can be seen in this figure, in sections where brightness is high, namely, where density is low, the level of density is the same even if levels of brightness are different. On the other hand, in sections where brightness is low, namely, where density is high, a phenomenon occurs in which the density jumps several levels when brightness is different by only one level.
The above explanation will now be supplemented with reference to the graph shown in FIG. 3, in which an original image density is plotted on a horizontal axis and the level of density converted from start signals by CCD input is plotted on a vertical axis. As can be seen from this graph, the higher the density, the more difficult it is to determine the level of density. Therefore, even if a signal has a brightness level of 256, about 50% of the data is lost when the density is LOG converted.
As can be understood from this phenomenon, there is a problem in that the gradation of a high-density shadow section of an image cannot be accurately reproduced. In a highlighted section, there is a tendency that reproduced densities which are less than a density of 0.1 are apt to be lost.
This phenomenon causes smooth highlighted gradation to be quite poor. This results in a problem that light shades of color cannot be reproduced by multi-color reproduction.
When repeated copies are made from a generation copy used as a manuscript, the density data of the shadow and highlighted sections are considerably diminished. These problems are major factors contributing to overall image deterioration.